A lithographic apparatus is a machine that applies a desired pattern onto a target portion of a substrate. Lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In these circumstances, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern corresponding to an individual layer of the IC, and this pattern can be imaged onto a target portion (e.g., comprising part of, one or several dies) on a substrate (e.g., a silicon wafer) that has a layer of radiation-sensitive material (resist).
The imaging process typically includes the use of positioning devices to accurately place the reticle in a precise position so that the generated circuit pattern is correctly placed onto the individual layer of the IC. The precise positioning of the reticle depends on several factors, such as maintaining an optimal focal distance between the reticle and the layer of the IC. Positioning errors parallel to the radiation path between the reticle and the layer result in worsened image focus. Focus positioning errors can result from many sources, such as a non-flat substrate of the IC. These errors can result in spatially varying defocus across the IC and negatively affects the quality of the circuit pattern image. As the wavelengths used to produce the circuit pattern images continue to get shorter, thus allowing for smaller circuit patterns, any distortion introduced in the imaging process becomes increasingly critical.